1. Field of the Invention
The invention relates to all over pin hole perforations in the plies for multiwall bags and more particularly to apparatus for producing all over pin hole perforations in a skip pattern. A skip pattern is desirable for preventing interply paste from leaking through the perforations.
2. Brief Description of the Prior Art
Multiwall bags with all over pin hole perforations have been manufactured and used for many years. The purpose of the perforations is to allow air to escape while the bags are being filled (with the end product). The original method of perforating was to position a unit between the paper unwind stands and the mutiwall tubing machine, hereafter called the tuber; this unit consisted of a roll with protruding pins equidistant around the circumference, as well as laterally across the face of the roll--and geared to an adjacent roll having grooves cut circumferentially, corresponding in lateral position with the pins. The pile sheet would be pulled between these rolls, and friction would cause them to rotate, perforating the paper in an all-over pattern. The pile sheet was then separated in the tuber into individual plies for interply pasting, and recombined to form a tube. This resulted in a misalignment of the holes--from ply to ply--in the finished bag, which allowed air to escape while filling, but minimized product sifting.
In some later bag designs, all but the outside ply were perforated, allowing air to escape through the porous outside sheet but with no product sifting. In other designs the various plies would have different hole patterns and sizes. Some specifications called for perforating the sheets from inside out and others from outside in. The introduction of barrier films made the perforating process more difficult because of a tendency of punctures in film to reclose. Replacing the grooved roll with a brush roll sometimes facilitated the perforation of films. In most instances the new bag designs required the use of more than one perforating unit--sometimes a unit for each ply.
In all of these prior designs, from oldest to most recent, the perforations formed a continuous pattern--hence the name "all over pin hole perforations". This configuration has presented certain problems to the bag maker. All pasted multiwall tubes have interply pasting, consisting of rows of paste dots applied between plies at the tops and bottoms of the tubes, to hold them together. The paste application is timed to the same position on every bag. Frequently, it happens that paste is applied over pin hole perforations, resulting in paste leaking through the holes. Leaks to the inside of the finished tubes may stick them shut, such that they cannot be readily opened in the bottoming operation. Leaks to the outside of the tubes may result in their being stuck together in a finished stack.
The apparatus of the present invention overcomes the disadvantages of the prior designs by providing for skip perforation wherein the tubes for multiwall bags are all-over perforated except at the tops and bottoms of the tubes in the area where the plies are pasted together.